Over a dozen Pakistani air force planes have crashed in roughly the past 18 months, raising concerns about the health of an aging fleet that officials are struggling to upgrade because of a lack of funds.

A significant number of the air force's combat aircraft are nearly half a century old and have been called on in recent years to help the army fight a domestic Taliban insurgency that has killed thousands of people. This has added to the strain on a force that has historically focused on countering the threat from Pakistan's neighbor and archenemy, India.

Pakistan has turned to the US and China for help in modernizing its air force, but economic woes have strained the government's budget, even for the country's powerful military. Tension with the US over a host of issues, including the covert raid that killed Osama bin Laden last year, also hasn't helped.

Shahzad Chaudhry, a former senior Pakistani air force officer, said the number of planes that have crashed "would be worrying for any air force".

"If I were air force chief, I certainly wouldn't want to see over a dozen airplanes being lost," said Chaudhry. "Obviously we need to bring those numbers down."

At least 13 planes have crashed since May 2011, many because of technical problems, according to a record maintained by The Associated Press. The air force did not respond to request for comment on the crashes.

The most recent accident occurred on November 22, when a Dassault Mirage fighter jet crashed on a routine night training mission in central Punjab province, killing the pilot. The air force said at the time that it was investigating the cause of the crash, but eyewitnesses said the plane caught fire before it fell out of the sky.

Nearly half the planes that have crashed were decades-old Mirage fighters. They make up at least a quarter of the force's fleet of about 400 combat aircraft, according to the website GlobalSecurity.org. Many were built nearly 50 years ago and acquired from foreign militaries that had already retired them.

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